John Kaehny, an advocate for increased transparency in Albany, says the mayor is being investigated over a common practice used by other politicians.

To get around campaign finance laws that limit how much an individual can contribute to a candidate, donors can give money to political committees instead.

"This is about using a very heavily used and well-known loophole in campaign finance laws," Kaehny says. "It's a backdoor way to funnel very large amounts of money from big donors to candidates."

Some Republicans in the state Senate, like Sen. Terrence Murphy, who represents an area in the Hudson Valley, have accused de Blasio of funneling the money to Democratic challengers seeking to oust GOP incumbents.